Paper is made in a continuous process beginning in the forming section of a papermaking machine where a very dilute slurry consisting of about 99% water and 1% papermaking fibers is delivered at high speed from a headbox onto a moving forming fabric. Modern papermaking machines can be very wide, up to and exceeding about 400 inches (about 10 m) in width or more, and are capable of manufacturing paper product at speeds in excess of more than one mile per minute (1.6 km/min) at this width. The forming fabric, or fabrics in a so-called twin-wire papermaking machine, is/are intended to form the sheet, convey it through the forming section and allow for drainage of large volumes of fluid from the papermaking stock so that its consistency reaches about 25% papermaking fiber content at the end of the forming section. Drainage and sheet formation are enhanced by means of stationary foil blades, suction boxes and other devices which are located beneath and in sliding contact with the forming fabric or fabrics. High pressure showers are used to clean the fabric and assist in trimming the sheet. The very wet and loosely cohesive embryonic paper web is then transferred in a transfer process from the forming section to the press section where further water removal occurs by mechanical means as the sheet is pressed in a series of press nips while it is sandwiched between two press fabrics in each nip. Final water removal occurs in the dryer section where the web is further dried by evaporative means as it is transported over a number of heated drums while conveyed upon one or several dryer fabrics in succession.
It will be appreciated that the fabrics used in the papermaking process must be very rugged so as to withstand the abrasive wear, fabric tensions, elevated temperatures, and chemical environment to which they are exposed. In the forming section particularly, the fabrics are exposed to relatively high tensions and levels of wear as they move across and around various stationary fabric supporting elements and rolls while transporting a layer of papermaking stock. Forming fabrics must also possess a very fine, smooth papermaking surface structure so that the sheet that is formed and conveyed upon them is as uniform as possible and is devoid of any marking that could be imparted by the fabric and its seam.
Because of these various and competing requirements, it will be apparent that selection of both the fabric weave pattern and the component materials, particularly the monofilaments from which the fabrics are formed, is of great importance with respect to the stability, durability and papermaking utility of the fabric. Forming fabrics are currently woven using thermoplastic monofilaments with relatively small diameters. For example, circular monofilaments used in such fabrics will have diameters typically in the range of from about 0.10 mm to about 0.5 mm (0.039 to 0.193 in) and will be woven at mesh counts on their paper side surface (i.e. number of MD yarns per unit fabric width) of from about 30-40 up to about 90-110 yarns per inch (from about 12-16 up to about 35-43 yarns/cm). It is also known to use generally rectangular, square, oval and similar cross-sectional yarn shapes in forming fabrics.
Forming fabrics are often double layer woven structures designed to provide the requisite fine and smooth papermaking surface that is mounted upon a much more rugged, stable structure which, when in use, is in sliding contact with the stationary elements of the forming section. However single layer, triple layer and other similar constructions are known and used (see e.g. PAPTAC [Pulp and Paper Technical Association of Canada] Data Sheet G-18, Rev. May 2005, entitled “Weaves of Papermaking Wires and Forming Fabrics” for examples of other forming fabric weave structures in which the monofilaments of the present invention would be applicable).
The monofilaments from which forming fabrics are frequently formed are generally extruded from a thermoplastic polymeric material, commonly a polyester such as poly(ethylene terephthalate) or PET, poly(butylene terephthalate) or PBT, poly(ethylene naphthalate) or PEN, and the like. Various polyamides (e.g. nylon-6. nylon 6/12, etc.) and polymer blends (e.g. polyester-polyurethane) are also known and used to provide the fabrics with the requisite wear resistance, stability, strength and general durability. It is an important characteristic of the yarns employed in the manufacture of papermaker's forming fabrics that they possess physical properties amenable to weaving, heatsetting and similar processes employed in the formation of the textile.
In recent years, a number of polymeric materials have been developed and used with varying degrees of success in the construction of papermakers fabrics intended for the forming, press or dryer sections of the papermaking machine. These materials include, for example, various polyamides, polyphenylene sulfide (PPS), polyetheretherketone (PEEK), liquid crystal polymers (LCP), and numerous polymer blends that have been engineered so as to provide specific physical properties that would render them suitable for papermaking fabric applications. Of these various polymers, polyesters have proven to be the most successful materials for these applications due, in part, to their resistance to dimensional change, chemical degradation and abrasion, as well as their weaving and processing characteristics, all of which are factors important to maximize the wear life and overall quality of papermaking fabrics.
Recently, it has been proposed to use yarns formed from poly(ethylene naphthalate), or PEN, in papermachine fabrics, in particular as the warp yarn material which is oriented in the length or machine direction of the fabric, due to its high modulus and other desirable physical properties. Modulus, also referred to as elastic modulus and expressed in pounds per linear inch (or kg/mm2), generally refers to the resistance of a yarn or fabric to stretch or distortion under tension. Papermakers forming fabrics must be stretch resistant, and have a relatively high tensile strength so as to resist catastrophic failure due to the tensions caused by the drag load and other frictional forces to which they are exposed. Forming fabrics intended for the production of kraft and similar high basis weight brown paper grades must have a higher tensile strength than fabrics intended for the manufacture of tissue (lighter basis weight) products due to the dewatering and formation forces to which each product is exposed. However, regardless of the grade of paper for which the fabric is intended, manufacturers of forming fabrics will strive to employ yarns which have a high tensile strength and elastic modulus and are thus resistant to stretch. PEN is a good candidate material because it is known to be resistant to elastic distortion and also possesses good tensile strength characteristics.
However, there are various problems associated with the use of monofilaments comprised of 100% PEN polymer (also referred to as “pure” PEN) in the manufacture of forming fabrics. For example, it is difficult to reliably extrude this material in monofilament form to provide a yarn product having acceptable diameter variation along its length. Strand diameter variations will cause problems with forming fabrics, including non-uniform drainage of the papermaking slurry, non-uniformities in the paper product being formed, irregularities in the tensile strength of the fabric, and so on. Another problem that has been observed in monofilaments formed from PEN relates to what is referred to as “notch sensitivity”. The term notch sensitivity means that surface imperfections, such as dents or scratches or similar surface deformities, will quickly propagate and cause yarn breakages while under tension, making such monofilaments particularly difficult to weave on modern high speed looms. In addition, the material is difficult to process and special fabric processing parameters (such as heatsetting temperatures, dwell times and tensions) must be used because PEN yarns are usually highly oriented (stretched) to provide their correspondingly high elastic modulus strength. This makes the PEN monofilaments difficult to crimp when interwoven with a relatively softer weft material. Fabrics containing pure PEN monofilaments as warp materials must be heatset for longer periods than fabrics woven using other polyesters such as PET. This lengthy processing is further exacerbated due to the comparatively lower force of shrinkage (FOS) generated by pure PEN monofilament as compared to PET polyester, for example. Force of shrinkage relates to the tendency of a yarn to shrink during application of heat, such as in a fabric heatsetting process. Due to the low FOS of pure PEN monofilaments, fabrics containing these yarns must be stretched to a greater degree in the heatsetting process than fabrics made from PET polyester monofilament yarns so as to transfer crimp to the cross-machine direction weft yarns and develop acceptable fabric properties. In addition the cost per unit weight of the PEN polymer resin from which the monofilaments are formed is generally higher than that of a comparable PET resin, making woven products formed from the PEN resin more expensive to produce.
Due to these various problems, and other known deficiencies of fabrics and monofilaments formed from pure or known blends of PEN, it would be highly desirable if a thermoplastic monofilament were available for use in papermakers fabrics which monofilament was dimensionally stable over a wide range of environmental conditions, provided sufficient levels of abrasion resistance so as to enhance the longevity of the fabric, offered good weaving properties so that it could be incorporated into the fabric structure, provided sufficiently high elastic modulus to impart stretch resistance to the fabric, could be reliably extruded with the required shapes and dimensions, was capable of being processed in a fabric at temperatures and tensions similar to PET, and which was available at a reasonable price.